1. Field of the Invention
This invention relates to selectively engageable and disengageable couplings and, in particular, to an armature for use in such couplings that reduces noise generated during burnishing of the coupling.
2. Disclosure of Related Art
A conventional selectively engageable and disengageable coupling for an automotive compressor or similar application includes a rotor assembly, an armature assembly, and means for selectively urging the armature assembly into engagement with the rotor assembly. The rotor assembly may include a rotor having an inner pole ring rotatably supported on the outer race of a bearing that is mounted to the compressor, an outer pole ring configured for engagement with an endless belt by which the rotor is driven, and a friction face connecting the inner and outer pole rings. The armature assembly may include a hub mounted to the compressor drive shaft, a resiliently flexible spider telescopically mounted on the hub, and an armature disc held by a radially outer portion of the spider. The urging means may include an electromagnetic coil disposed within a magnet core supported between the inner and outer pole rings of the rotor. Excitation of the coil creates a magnetic circuit among the magnet core, rotor, and armature whereby the armature is drawn into frictional engagement with the rotor and torque is transferred from the rotor to the compressor drive shaft.
During burnishing of conventional couplings (i.e., during the initial on-off cycles of the coupling wherein the rotor and armature surfaces are ground upon engagement until the surfaces evenly match), galling typically occurs as the metal alloy surfaces of the rotor and armature engage one another. This results in a relatively high level of noise generated by the coupling during burnishing. Moreover, galling can cause the coupling to generate an inconsistent torque and may cause the armature to xe2x80x9cslipxe2x80x9d relative to the rotorxe2x80x94potentially resulting in a failure of the armature assembly.
The armatures in some conventional couplings have been hardened through the addition of metal plates or a nitriding process. Galling during burnishing of couplings incorporating such armatures is reduced and/or eliminated because of the hardened surface of the armature. The hardened surface, however, makes it difficult to achieve a sufficient torque in the coupling. Further, the hardened surface of the armature produces another type of noise (similar to a knife on glass).
There is thus a need for an armature for a selectively engageable and disengageable coupling that will minimize or eliminate one or more of the above-mentioned deficiencies.
The present invention provides an armature for a selectively engageable and disengageable coupling and a method of making the armature.
An armature in accordance with the present invention includes a substantially annular body that is disposed about a first axis. One side of the body may be hardened through a nitriding process to reduce galling during burnishing of a coupling including the armature. The body of the armature is configured so that the thickness of the body increases as the distance from a line extending perpendicular to the first axis increases. In particular, the line may be taken across a diameter of the body and the thickness of the body may vary along each cross-section taken along the line from a minimum thickness at the innermost portion of the cross-section to a maximum thickness at the outermost portion of the cross-section.
A method of forming an armature for a selectively engageable and disengageable coupling in accordance with the present invention includes the step of forming a substantially annular body wherein the body is disposed about a first axis. The method also includes the step of hardening a first side of the body through, for example, a nitriding process. Finally, the method includes the step of deforming the first side of the body such that a thickness of the body varies as the distance from a line extending perpendicular to the first axis increases. The deforming step is preferably accomplished by placing a grinding wheel at a predetermined angle and conveying the body past the grinding wheel in a direction perpendicular to the first axis. The grinding wheel forms a trench in the first side of the body along a diameter of the body. As a result, the thickness of the body will vary along each cross-section taken along the diameter from a minimum thickness at the innermost portion of the cross-section to a maximum thickness at the outermost portion of the cross-section.
An armature in accordance with the present invention represents a significant improvement as compared to armatures found in conventional selectively engageable and disengageable couplings. First, because the armature is hardened, galling during burnishing of the coupling is reduced, thereby reducing noise during the burnishing process. The reduction in galling also enables the coupling to generate a more consistent torque during the burnishing process and reduces the likelihood that the armature will xe2x80x9cslipxe2x80x9d relative to the rotor and that the armature assembly will fail. Second, the deformation of the armature body roughens the hardened surface of the body, thereby increasing the initial torque of the coupling and reducing the time required for burnishing. The roughened surface also reduces the xe2x80x9cknife on glassxe2x80x9d noise that can result from the engagement of the rotor and the hardened surface of the armature. Third, the variation in thickness of the armature body produced by the deformationxe2x80x94whereby the thickest portions of the armature body are located along the radially outer portions of the bodyxe2x80x94ensures that the radially outer poles of the armature body will be the first portion of the armature to engage the rotor.
These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.